This invention relates to a process for strengthening the grain boundaries of a component made from a Ni based superalloy.
Single crystal turbine components are manufactured from Ni based superalloys using a directional solidification technique. Casting a large perfect single crystal component is in practice extremely difficult, with most such components being subject to defects such as grain boundaries, freckles, equiaxed stray grains and microporosity among others. These defects generally weaken the components at high temperature, resulting in an inability to maintain a desired life of the component or a desired temperature of operation of the turbine, which will ensure high turbine efficiency, without risking component failure initiated at the defect. However, to demand nothing but perfect single crystal articles from a foundry would incur a very high scrap rate and concomitant high per-part costs. Thus, the industry trend is to accept as many defects as possible without compromising the lifetime or operating temperature of the components.
One of the most prevalent defects is grain boundaries, which are particularly harmful to the single crystal components high temperature properties. Grain boundaries are regions of high local disorder of the crystal lattice as they are the locations at which neighboring grains must join together despite a certain misorientation between their lattices. The greater the misorientation, the greater is the disorder (concentration of dislocations) in the grain boundary required to facilitate the fitting together of the two grains. This disorder is directly related to the behavior of the grain boundary at higher temperatures, making it weaker with respect to the bulk material inside the grains as temperature increases above the xe2x80x9cequicohesive temperaturexe2x80x9d, which is generally 0.5Tm where Tm [K] is the melting point of the material.
This weakening effect has been clearly established in patent GB-A-2,234,521. The FIG. 4 of the disclosure GB-A-2,234,521 plots stress rupture strength at 871xc2x0 C. tested across grain boundaries of various degrees of misorientation. Note that for the xe2x80x9cbasexe2x80x9d material (conventional single crystal alloy) there is a sharp drop in the properties when the misorientation exceeds about 6xc2x0. The trend is also shown in xe2x80x9cSuperalloy 1996xe2x80x9d (Ed. R. D. Kissinger et al.,The Minerals, Metals and Materials Society 1996) for the alloy Rene N for stress rupture at 1600xc2x0 F. and 1800xc2x0 F. The sudden weakening of the single crystal article containing grain boundaries of misorientation higher than 6xc2x0 has led to the clear specification that no misorientations above 6xc2x0 are acceptable.
In the past, Ni based superalloys cast to give an equiaxed grain structure or columnar-grained structure were fortified with elements such as C (carbon) and B (boron) which are known grain boundary strenghtheners, as they cause the precipitation of carbides and borides, which are stable at high temperatures, on the grain boundaries. In addition the presence of these elements in solution in the grains and along the grain boundaries slows down diffusion processes at high temperatures, which is a major source of grain boundary weakness.
It was discovered early in the evolution of single crystal alloys that the presence of significant quantities of C and B in the alloys prevented the maximum strength of the alloys from being achieved, for three main reasons:
1. with high levels of carbon, elongated carbides tend to form between dendrites during directional solidification, and these can be crack starters during service,
2. C and B increase the amount of eutectic in the as-cast article, which degrades LCF (Low cycle fatigue) and creep properties and
3. C and B dramatically lower the melting point of the alloy. At the levels present in DS alloys, the incipient melting point is often lower than the gamma prime solvus temperature, which prevents a complete solutioning of gamma prime and re-precipitating at the desired size range, and prevents the complete solutioning of gamma/gamma prime eutectic. This can have a dramatic effect on LCF and creep properties.
For these reasons, C and B levels were kept extremely low in the first generation single crystal superalloys. However, patent GB-A-2,234,521 shows that high temperature properties can be maintained with levels of carbon higher than in conventional single crystal alloys but lower than in previous art DS alloys. The invention disclosed in GB-A-2,234,521 has allowed to raise the defect toleration specification from 6xc2x0 to 12xc2x0 for the new alloy, hence the concept of xe2x80x9cdefect tolerant alloyxe2x80x9d.
It is recognized that the general trend in the most recent generation of patented single crystal Ni based superalloys is towards levels of C from 250 ppm to 600 ppm. Recent patents, e.g. U.S. Pat. Nos. 5,455,120 and 5,399,313, discloses a range from 200 up to 700 ppm C, the U.S. Pat. No. 5,482,789 a range from 0-600 ppm C, while the recent patent U.S. Pat. No. 4,719,080 from United Technologies discloses a range from 0 up to 450 ppm C. A range of 200-400 ppm C is also disclosed in the patent U.S. Pat. No. 5,759,301 for a Ni based single crystal superalloy.
Although the highest content of carbon disclosed in the above mentioned documents is 700 ppm, publications indicate that the commercially used versions of these alloys contain 500 ppm C. The reasons for this limit, despite the knowledge that higher amounts of carbon would further increase grain boundary strength, are described above. No solution for this problem of the current art has ever been disclosed. The current art improves grain boundary strength in single crystal alloys exclusively by having carbon as an alloying element in the casting alloy, so that carbides form along the grain boundaries during solidification due to segregation of carbide forming elements to the grain boundaries.
The patent U.S. Pat. No. 5,598,968 discloses a method of using carburization to precipitate carbides in the surface layer of a superalloy article in order to prevent recrystallization during subsequent heat treatment.
However, U.S. Pat. No. 5,598,968 is specifically addressing articles that have been or will be cold worked, and the carburization depth desired is associated with the depth of cold working of the surface. Patent U.S. Pat. No. 5,598,968 also discloses only for the carburization as being part of a process in which there is a) cold work and b) subsequent heat treatment during which recrystallization may occur. It is recognised that such a heat treatment, which may cause recrystallization, must approach the gamma prime solvus temperature of the Ni based superalloy.
The desired effect of patent U.S. Pat. No. 5,598,968 is obtained once the surface of the superalloy is carburized and caused to grow a dispersion of carbides up to a certain predetermined depth in the surface of the component. No mention is made of grain boundaries.
Note also that patent U.S. Pat. No. 5,598,968 implies that the surface carbide dispersion will be left in the article during service. It is disclosed that the carbides help to prevent the formation of the undesirable Secondary Reaction Zone (SRZ)xe2x80x94which forms during service, and obviously must be left on for the heat treatment. Those skilled in the art recognise the SRZ is a problem to high Re alloys in which undesirable precipitates form at various locations but particularly at the surface layer of a coated component. SRZ will precipitate after several thousand hours of services, hence the need to leave the carbide dispersion in the surface layer during the service life of the part.
It is industry standard to use grit blasting at least twice during normal routing: once to clean the surface in preparation for grain etching and again to prepare the surface for fluorescent penetrant inspection. As each grit blasting operation removes from 10-25 xcexcm of surface material, by definition most of the surface carburization as described in patent U.S. Pat. No. 5,598,968 will be removed. Importantly, as the cold work to be done to the articles described in the patent U.S. Pat. No. 5,598,968 can only be done on the outside, and the carburization is required only where the material has undergone cold work, it is clear that the intention is not to obtain or control for carburization from inside the component when the component is hollowxe2x80x94as in turbine blades and vanes with cooling passages. It would be nearly inconceivable that patent U.S. Pat. No. 5,598,968 would lead to a full carburization of the grain boundaries with the small one-sided surface penetration specified in the patent.
It is the object of the present invention to find a novel method of treatment of Ni based superalloy articles, particularly for but not limited to single crystal articles, in order to introduce C or B into the grain boundaries while these articles are in the solid state, which means after casting. Carbon or Boron (or both) shall be introduced into the grain boundaries of the article so that the grain boundaries show at least higher C and/or B levels than normally observed after casting, using an alloy composition with the maximum C or B levels as specified in the manufacturing instructions for the alloy for use in that component in particular with no upper limit. This invention follows the finding that the carbides formed by carburization offer similar grain boundary strengthening properties as those cast into the article using the current art without the detrimental effects of adding more carbon to the alloy prior to casting.
The method of processing may include a means of introducing carbon and/or boron simultaneoulsy on the outer working surface of the article as well as on the inner working surface, e.g. the cooling configuration of a turbine blade.
The desired effect of the present invention is to introduce carbon along the grain boundaries with no regards to effects at the surface. Rather than measuring the carburization effect in terms of width of carburization in the overall surface, the effect is measured as carburization only along the grain boundaries in the cast article. The desired depth of carburization in the present invention is decided by the physical design of the component and where the grain boundaries occur: That is, the wall thickness in which the grain boundary is found determines the depth of the grain boundary and hence the depth of carburization, not all grain boundaries in the part need be carburized, only those experiencing high loadings at high temperatures. This may be up to 3 or more mm in depth. Importantly, the carburization (and/or Boron enrichment) step may be carried out with no association to any other heat treatment which may cause recrystallization, and may be done before, during or after such a heat treatment.
In addition, one advantageous embodiment of the present invention is that the surface layer of carbides may be removed by chemical or mechanical means because this surface layer is inconsequential to the desired effect of carburization of the grain boundaries.
The precipiates are formed advantageously from the group of secondary carbides such as HfC, M23C6, M6C, M7C wherein M is a metal, preferred Cr.